We know that every $CFL$ has infinite configuration space. Due to this equality problem is undecidable. But why finiteness property is decidable inspite having infinite configuration space?


1 Answer 1


The language generated by a grammar with no useless symbols/productions is finite if and only if there is no non-terminal $A$ so that $A \Rightarrow^* \alpha A \beta$. This is easy to check.

  • $\begingroup$ $ \Rightarrow^*$ means arrow $ \Rightarrow$? $\endgroup$
    – Punia
    Oct 12, 2021 at 22:49
  • $\begingroup$ But why equality isn't decidable in same logic? $\endgroup$
    – Punia
    Oct 12, 2021 at 22:56
  • 1
    $\begingroup$ @Net, if at least one of the languages is finite, it is possible. How would you check in case both languages are infinite? $\endgroup$
    – vonbrand
    Oct 13, 2021 at 22:34

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